1. Field of the Invention
The present invention relates generally to centrifugal turbo-machines and, more particularly, to a centrifugal turbo-machine in which an axial thrust control member is configured to automatically control axial thrust generated by a difference between static pressures of front and rear ends of an impeller provided in a centrifugal pump or compressor, thus appropriately controlling axial thrust even if the axial thrust varies attributable to abnormal operation conditions.
2. Description of the Related Art
Generally, a centrifugal turbo-machine is a machine which applies kinetic energy (dynamic pressure) to fluid using reaction induced by rotation of an impeller and converts it into pressure energy (static pressure). A centrifugal pump, a centrifugal compressor or the like is a representative example of the centrifugal turbo-machine.
FIG. 1 is a sectional view showing the construction of a centrifugal turbo-machine 10 according to a conventional technique.
Referring to FIG. 1, the conventional centrifugal turbo-machine 10 which converts kinetic energy applied to fluid into pressure energy includes a rotating shaft 12, an impeller 13, a volute casing 11 and seals 14 and 15. The rotating shaft 12 is rotatably installed in the volute casing 11 and supported by a bearing 16.
The impeller 13 is fastened to the rotating shaft 12 and rotates along with the rotating shaft 12. The impeller 13 draws fluid using centrifugal force generated by rotation thereof.
The volute casing 11 defines therein a space into which fluid drawn by the impeller 13 flows. In the volute casing 11, dynamic pressure of drawn fluid is converted into static pressure. In other words, in the volute casing 11, kinetic energy of drawn fluid is converted into pressure energy.
The seals 14 and 15 reduce the amount of leakage of drawn fluid to increase the efficiency of the centrifugal turbo-machine 10. The seals 14 and 15 are positioned corresponding to the front and rear ends of the impeller 13.
The operation of the conventional centrifugal turbo-machine 10 having the above-mentioned construction will be explained below.
The impeller 13 rotates in the hermetically sealed volute casing 11 to draw fluid into the volute casing 11. Then, centrifugal force is generated by the impeller 13. Fluid is drawn into the volute casing 11 by the centrifugal force of the impeller 13. While the drawn fluid flows into the volute casing 11, dynamic pressure of fluid is converted into static pressure in the volute casing 11, thus producing pressure energy.
However, some of fluid drawn by the impeller 13 flows through gaps between the surface of the impeller 13 and the seals 14 and 15 rather than being drawn into the volute casing 11. Fluid passing through the gaps defined by the seals 14 and 15 differ in pressure from each other, thus generating axial thrust.
As shown in FIG. 1, the shapes of the front and rear ends of the impeller 13 differ from each other and the area of the gap between each end of the impeller 13 and its surrounding casing also have difference. Thus, pressures formed around the front and rear ends of the impeller 13 differ from each other. Furthermore, pressures around outlets of the seals 14 and 15 differ from each other. Therefore, axial thrust is generated in a direction from the rear end of the impeller 13 towards the front end thereof.
This axial thrust is applied to the rotating shaft 12 of the centrifugal turbo-machine 10. The force applied to the rotating shaft 12 is supported by the bearing 16 coupled to the impeller 13.
Here, in the case where appropriate intensity of axial thrust is applied to the rotating shaft 12, the bearing 16 can reliably support the rotating shaft 12. However, if excessive axial thrust is applied to the rotating shaft 12, the expected lifetime of the bearing 16 is reduced. If it exceeds a limit, the bearing 16 may be damaged.
Therefore, to prevent damage of the turbo-machine 10 and increase the lifetime of the bearing 16, the axial thrust should be successfully controlled. For this, a difference between static pressures applied to the front and rear ends of the impeller 13 must be reduced.
In the conventional technique, to reduce a difference between static pressures applied to the front and rear ends of the impeller 13, the area of gap between the impeller 13 and the volute casing 11 was changed by varying the diameters of the seals 14 and 15 provided around the front and rear ends of the impeller 13.
In detail, the conventional technique has used a method in which the intensity of axial thrust generated around the rear end of the impeller 13 is reduced by increasing the diameter of the seal 15 provided around the rear end of the impeller 13 which typically generates relatively large axial thrust. However, the method of reducing axial thrust by changing the diameter of the seal 15 requires much time and costs in manufacturing the turbo-machine, so that it is not economic.
Recently, in an effort to overcome the above problem of poor economy, a method of installing an axial thrust control member for controlling axial thrust in a turbo-machine has been developed.
FIG. 2 is a sectional view showing a turbo-machine 20 having an axial thrust control member 30 according to a conventional technique.
Referring to FIG. 2, the turbo-machine 20 having the axial thrust control member 30 can more economically control axial thrust, compared to the prior method of changing the diameter of the seal. However, if input values different from the input values it was designed for are applied to the turbo-machine 20 while it is being operated, an operational problem may be induced. Furthermore, there is a disadvantage in that the turbo-machine 20 may not be able to resist abnormal operation circumstances.
For example, in the case where a flow rate of fluid drawn into the turbo-machine 20 is less than the flow rate it was designed for, output pressure is increased and a pressure around the impeller 23 is also increased. Thereby, the entire axial thrust applied to the turbo-machine 20 is also increased.
Furthermore, if a design of a fluid supply system for operating the turbo-machine 20 is not appropriate or a loss of pressure of the fluid supply system is increased by penetration of foreign substances while the turbo-machine 20 is being operated, a flow rate of fluid drawn into the turbo-machine 20 becomes less than the designed flow rate and the axial thrust applied to the turbo machine 20 is increased.
In addition, in the case where the design of the impeller 23 or the volute casing 21 does not correspond to the designed flow rate, there is a probability of an increase in output pressure. This also is a factor of an increase in axial thrust.
Moreover, the axial thrust control member 30 cannot automatically control axial thrust while the turbo-machine 20 is being operated. Merely, the height of the rib 31 of the axial thrust control member 30 is determined to a degree capable of reducing axial thrust in consideration of the intensity of axial thrust expected to be generated while the turbo-machine 20 is being operated. Then, pressure of fluid drawn through the rear end of the impeller 23 is reduced by the resistant force of the rib 31, thus controlling axial thrust.
However, to effectively use the axial thrust control member, after a design flow rate of the turbo-machine 20 and output axial thrust are correctly checked, can the turbo-machine 20 be operated. Only then can the generation of expected axial thrust be appropriately controlled.
Furthermore, a problem in an increase of axial thrust exceeding an expected value because of the above several reasons cannot be controlled by the axial thrust control member 30. In this case, in the same manner as the prior turbo-machine 10 having no axial thrust control member, the bearing 26 may be damaged with the result that the lifetime of the turbo-machine is reduced.
As such, in the conventional turbo-machine, when pressure around the seals 24 and 25 is increased over an expected value, the axial thrust control member 30 cannot exhibit its intended function. To improve this, a precise measure of axial thrust is indispensably conducted before the machine is operated. If unexpected measurement results are produced, the design of the axial thrust control member 30 must be revised, or it must be newly manufactured or installed.